The invention relates to a voltage controlled oscillator (VCO) circuit which can be controlled by an input voltage to operate over a large frequency range. Typically, such circuits employ a capacitor which is charged and discharged by controlled currents thereby producing a fundamental frequency that is determined primarily by the capacitor value. Such oscillator circuits are useful in FM applications and in phase-locked loop applications. The present circuit is intended for use in integrated circuit (IC) chips. While it can be a stand-alone chip, it is more common to include the VCO along with other circuits on a complex circuit chip. It is intended that the IC form will be the well known monolithic silicon, epitaxial, planar, PN junction isolated structure. Since all of the transistors in the circuit are NPN devices, the frequency of operation can be relatively high.
In VCO circuits, the frequency determining capacitor is typically an off-chip component that is attached, by the user, to the package pins. This is done primarily because the user ordinarily wants to control the circuit frequency in accordance with his system design and does not want to be limited to an arbitrary frequency range.